1. Field of the Invention
The present invention relates to a modified polypropylene and process for producing same. More particularly, this invention is concerned with a modified polypropylene which is a polypropylene having a value of racemic diad fraction [r] in a specific range and modified with a specific compound, e.g., (meth)acrylic acids, and their derivatives or styrene derivatives, and process for producing same. These polymers enable to improve three properties such as, solubility in organic solvents, affinity for polypropylene-based materials and thermal stability, well in the balance.
2. Description of the Prior Art
Conventional polyolefins, e.g., polypropylene and polyethylene, show little affinity for other polymers, e.g., styrene, acrylic and vinyl acetate resins, because of their high crystallinity and non-polar characteristics, causing difficulties in blending polyolefins with other polar materials, coating and adhesion, and printing polar materials on polyolefin-based materials. These are problems for conventional polyolefins.
Development of modified polypropylenes having good balance in the three properties of solubility in solvents, affinity for polypropylene-based materials and thermal stability is needed to solve the above problems.
Polymers of highly crystalline polypropylene incorporated with a functional group, e.g., chlorine or maleic anhydride, have been developed. However, these polymers with a functional group, in general, are insufficiently soluble in solvents, causing problems of difficulty in handling.
Therefore, the base polymers, e.g., low-molecular-weight isotactic polypropylene and copolymer of propylene with another α-olefin, have been studied to improve their solubility.
As the above base polymers, for example, Japanese Patent Laid-open Publication No.11-100412 proposes an isotactic polypropylene having narrow molecular weight distribution and high stereoregularity, so that the polymer has a very high stiffness and heat resistance. Japanese Patent Laid-open Publication No.2001-146565 proposes a resin composition containing a non-crystalline polyolefin (atactic polypropylene or propylene-α-olefin copolymer) grafted with a radical, polymerizable, unsaturated compound. This compound may have a functional group.
However, an isotactic polypropylene of high stereoregularity, e.g., that disclosed by Japanese Patent Laid-open Publication No.11-100412, has generally insufficient solubility in solvents, even if its molecular weight is low. According to Japanese Patent Laid-open Publication No.2001-146565, the resin composition has excellent solubility, inter-layer adhesion to top coat and weather resistance. However, it still fails to show satisfactory characteristics, because it is low in thermal stability and solubility in solvents when the polyolefin is an isotactic polypropylene, and not sufficient in affinity for polypropylene materials when the polyolefin is a propylene-α-olefin copolymer.
Japanese Patent Laid-open Publication No.2-41305 proposes a highly crystalline, syndiotactic polypropylene. The main chain structure of the polypropylene is mainly (80% or more, in particular 95% or more) composed of racemic diad(s). Japanese Patent Laid-open Publication No.4-348114 proposes modification of syndiotactic polypropylene. However, the polypropylene is not sufficiently soluble in solvents at room temperature, resulting from use of highly crystalline syndiotactic polypropylene.
It is discussed, e.g., by “Macromol. Rapid. Commun.,” H. Mori et al., 18, 157 to 161 (1997), that syndiotactic polypropylene has higher thermal stability than isotactic polypropylene. Moreover, Japanese Patent Laid-open Publication No.4-348114 also discusses that syndiotactic polypropylene has higher thermal stability than isotactic polypropylene, because the former shows a smaller molecular weight decrease during the modification process.